The conventional method of manufacturing integrated circuits involves the photolithographic replication of patterns from a mask onto a silicon wafer surface which has been treated with a photoresist material. Each such step is followed by conventional procedures such as developing, plating, etching, etc. These steps may be repeated a number of times on a single wafer, each pattern overlying patterns previously applied. With increasing miniaturization, the pattern elements have become smaller and smaller to the extent that pattern resolution is limited by the wavelength of light. This has already led to use of the shorter wavelengths of the ultraviolet spectrum. The successor technology appears to be X-ray lithography.
One of the problems which will arise in the use of X-ray lithography is that the masks are likely to be opaque to visible radiation. This would essentially rule out the use of conventional optical proximity aligners for mask to wafer alignment. There are several reports in the literature of X-ray alignment schemes for opaque masks. (J. H. McCoy and P. A. Sullivan, "Precision Mask Alignment for X-ray Lithography", Electron & Ion Beam Sci. and Tech.-7th Intnl. Conf. Proc., 536 (1976).) In one, X-rays are transmitted through marks in the mask and substrate and, in another, X-rays are transmitted through marks in the mask and excite fluorescent X-rays from substrate alignment marks. Many of these techniques require undesirable heavy metal alignment marks in the substrate or are limited to specific source-substrate combinations. Accordingly, it is a primary object of the present invention to provide method and apparatus for aligning an opaque mask pattern with a wafer pattern. Other objects, features, and advantages will become apparent from the following description and appended claims.